feat(11.1-01): verify src= code blocks and add summer docs:sync

- src= fences name a file, a Go declaration or Example body, or a docs:start region
- confinement: relative clean paths inside the root, no dotfiles or .env,
  no nested go.mod modules, Examples need // Output:, test regions must run
- a drifted or missing snippet is a problem, so docs:build writes nothing
- docs:sync rewrites drifted fence bodies in place
- fences render in figure.code with a source caption; .md fences keep only the language
- bonfire ExampleCall is the first verified example, shown in setup/installation

internal/docsite/load.go

@@ -179,6 +179,11 @@ func assemble(opts Options) (*site, []Problem, error) {
 	if err != nil || s == nil {
 		return nil, problems, err
 	}
+	sp, err := s.checkSnippets()
+	if err != nil {
+		return nil, nil, err
+	}
+	problems = append(problems, sp...)
 	if len(problems) == 0 {
 		rp, err := s.render()
 		if err != nil {